Snifit or Whifit takes places in W3-10,
You won't be able to continue through to Stump Glade until you complete the next 3 worlds. I think that a descriptive walkthrough of each level required to get to this point is beyond the scope of this question, so I will instead point you in the right direction.
Wiggler's Tree House. Recall that there are 3 stumps in the room where you find Wiggler -- on top of each stump is a Comet Piece, but some are inaccessable because their scraps have been ripped out of the world. If you've completed through W3-6, then you should have the appropriate scrap to fix one of the stumps.
If you do not have the appropriate scrap to continue on, you may find it in W3-5,
Loop Loop River. It is in the middle island in the first area of the level, stuck to a small hedge.
Paperize to fix the stump, and obtain the Comet Piece. On a more general note, the Comet Pieces in Wiggler's Tree House are the key to continuing on to the remainder of World 3.
By obtaining the Comet Pieces in the next 3 worlds and completing the objective in Wiggler's Tree House, you should be able to continue through World 3.